Photovoltaic DC-to-AC power converter and control method

ABSTRACT

An apparatus and method of control for converting DC (direct current) power from a solar photovoltaic source to AC (alternating current) power. A novel DC-to-AC power converter topology and a novel control method are disclosed. This combination of topology and control are very well suited for photovoltaic microinverter applications. Also, a novel variant of this control method is illustrated with a number of known photovoltaic DC-to-AC power converter topologies. The primary function of both control methods is to seek the maximum power point (MPP) of the photovoltaic source with novel, iterative, perturb and observe control algorithms. The control portion of this invention discloses two related control methods, both an improvement over prior art by having greatly improved stability, dynamic response and accuracy.

FIELD OF THE INVENTION

The present invention relates to an electrical power converter used witha solar photovoltaic source to condition and couple the DC energy of thesolar photovoltaic source to the AC lines of an electric utility.

BACKGROUND OF THE INVENTION

Most of today's solar photovoltaic (PV) power sources are utilityconnected. About 75% of these installations are residential rooftopsystems with less than 2 kW capability. These systems typically comprisea number of PV modules arranged in series configuration to supply apower converter, commonly called an inverter, which changes the directcurrent (DC) from the modules to alternating current (AC) to match thelocal electrical utility supply.

There is a difficulty with small solar power systems on residentialrooftops. Gables and multiple roof angles make it difficult on somehouses to obtain enough area having the same exposure angle to the sunfor a system of 2 kW. A similar problem arises where trees or gablesshadow one portion of an array, but not another. In these cases the DCoutput of the series string of modules is reduced to the lowest currentavailable from any cell in the entire string. This occurs because the PVarray is a constant current source unlike the electric utility, which isa constant voltage source.

An inverter that economically links each PV module to the utility gridcan solve these problems as the current limitation will then exist onlyon the module that is shaded, or at a less efficient angle and does notspread to other fully illuminated modules. This arrangement can increasetotal array output by as much as two times for some configurations. Sucha combination of a single module and a microinverter is referred to as aPV AC module. The AC output of the microinverter will be aconstant-current AC source that permits additional units to be added inparallel.

PV AC modules now available suffer poor reliability owing to earlyfailure of the electrolytic capacitors that are used to store the solarcell energy before it is converted to AC. The capacitor aging is adirect consequence of the high temperature inherent in rooftopinstallations.

The electrolytic capacitors in the power circuit perform two functions.First, the capacitors hold the output voltage of the PV modules close tothe maximum power point (MPP) output despite variations in sunlight,temperature or power line conditions and second, the capacitors storeenergy at the input and even out the DC voltage variations at thepower-line frequency that result from changing the DC to AC. Thesefunctions place an additional stress on the capacitor causing internalheating that adds to the effects of high external temperature.

The high temperature reached by PV system inverters is a naturalconsequence of their outdoor mounting. This requires a rainproofenclosure that complicates the heat removal process. The coincidence ofmaximum power throughput and losses at exactly the time of maximumheating by the sun on both the enclosure and the ambient air exacerbatesthe condition.

Existing inverter topologies have made the electrolytic capacitor anintegral part of the inverter circuit because of the high capacitancevalue required to store energy from the PV module. If high capacitanceis required, the only economic choice is the electrolytic capacitor.Plastic film capacitors are recognized as superior in agingcharacteristics, but are much more expensive for the same capacitance.Thus, a means to avoid use of electrolytic capacitors can contribute tothe reliability of PV power sources.

SUMMARY OF THE INVENTION

The invention is a novel DC-to-AC power converter topology and a novelcontrol method that makes this combination of topology and control verywell suited for photovoltaic microinverter applications. Also, a novelvariant of this control method is disclosed for application with anumber of known photovoltaic DC-to-AC power converter topologies. Theprimary function of both control methods is to seek the maximum powerpoint (MPP) of the photovoltaic source with iterative, perturb andobserve algorithms. The control portion of this invention discloses tworelated control methods, both an improvement over prior art by virtue ofhaving greatly improved stability, dynamic response and accuracy.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 shows the preferred embodiment of the power converter controlmethod.

FIG. 2 shows the prior art power converter control method.

FIG. 3 shows an alternate embodiment of the power converter controlmethod.

FIG. 4 shows the preferred embodiment of the power converter electricalcircuit topology to be used with the control method illustrated in FIG.1.

FIG. 5 shows a bi-polar boost, prior art inverter electrical topology tobe used with the control method illustrated in FIG. 1.

FIG. 6 shows a polyphase prior art inverter electrical topology to beused with the control method illustrated in FIG. 3.

FIG. 7 shows an H-bridge with transformer, prior art inverter electricaltopology to be used with the control method illustrated in FIG. 3.

DETAILED DESCRIPTION OF THE PRIOR ART

FIG. 2 illustrates the control system for a conventional photovoltaic(PV) DC-to-AC power converter. This power converter has a pulse widthmodulated, voltage regulating boost stage and a pulse width modulated,current regulating buck stage. Sinusoidal reference 62 follows AC line90 voltage and frequency. AC line current reference 63 is generated bymultiplying sinusoidal reference 62 by scaling factor 66. Actual AC linecurrent 64 is compared to AC line current reference 63 to create errorsignal 65. Error signal 65 drives buck stage 60 as part of this servoloop. Current 41 and voltage 42 of PV source 10 are sensed andmultiplied to provide 43, a measure of PV source 10 output power.Scaling factor 66 is periodically adjusted to determine the amount ofenergy sourced onto AC line 90. A control means is used to periodicallyperturb (45) scaling factor 66 and observe (44) the effect on PV outputpower 43. If an increase in scaling factor 66 results in an increase inPV power 43, scaling factor 66 is incrementally increased every perturbcycle until an increase in scaling factor 66 results in a decrease in PVpower 43. This is how the maximum power point (MPP) of PV source 10 isestablished. Boost stage 40 is transparent to this perturb and observefunction and serves as a typical voltage regulator to maintain thevoltage at energy storage capacitor 50 at a regulated voltage higherthat the peak voltage of the AC line. Fixed reference voltage 48 iscompared to feedback voltage 49 creating error signal 47 to drive booststage 40. In some inverters designed to work with PV voltages higherthan the peak AC line voltages, boost stage 40 is not required.

The problem with this prior art control method is instability and poordynamic response. If current reference 63 requests a current andtherefore power to be delivered into the AC line that PV source 10cannot supply, the control loop becomes unstable, PV source 10 voltagecollapses and cannot be recovered without restarting the power converterand the perturb and observe process. This prior art control method isunstable when operating on the lower-voltage side of the PV sourcemaximum power point. The maximum power point of a photovoltaic sourceusually changes slowly but moving cloud cover, wind gusts and partial,momentary PV source shadowing can abruptly push the maximum power pointinto an unstable region for this control method.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

FIG. 1 illustrates the preferred embodiment of the control arrangementfor the DC-to-AC power converter. This power converter has a pulse widthmodulated, voltage regulating boost stage and a pulse width modulated,current regulating buck stage. Sinusoidal reference 62 follows the ACline voltage and frequency. AC line current reference 63 is generated bymultiplying sinusoidal reference 62 by scaling factor 61. Scaling factor61 is a measure of the voltage on energy storage capacitor 50. Actual ACline current 64 is compared to AC line current reference 63 to createerror signal 65. Error signal 65 drives buck stage 60 as part of thisservo loop. The net effect is the voltage on energy storage capacitor 50is regulated by the amount (the amplitude) of current 64 sourced into ACline 90. Current 41 and voltage 42 of PV source 10 are sensed andmultiplied to provide 43, a measure of PV source 10 output power. PVreference voltage 46 is the desired operating point of PV source 10 andis compared to actual PV source 10 voltage 42 in a servo loop whereerror voltage 47 drives boost stage 40. A control means is used toperiodically perturb (45) PV reference voltage 46 and observe (44) theeffect on PV output power 43. If an increase in PV reference voltage 46results in an increase in PV power 43, PV reference voltage 46 isincrementally increased every perturb cycle until an increase in PVreference voltage 46 results in a decrease in PV power 43. This is howthe maximum power point (MPP) of PV source 10 is established. If theirradiance on and the temperature of photovoltaic source 10 aresubstantially stable. PV source voltage 42 will dither a small amountabout the MPP of PV source 10. This control method provides stableoperation at any PV source operating point. As conditions change thecontrol circuit will track and seek a new MPP.

FIG. 3 illustrates an alternate embodiment of the control arrangementfor the DC-to-AC power converter. This power converter has a pulse widthmodulated, current regulating buck stage and no boost stage. Sinusoidalreference 62 follows the AC line voltage and frequency. AC line currentreference 63 is generated by multiplying sinusoidal reference 62 byscaling factor 67. Scaling factor 67 is the error signal or differencebetween the actual operating voltage 42 of PV source 10 and PV referencevoltage 46, the desired operating voltage of PV source 10. A fixedoffset voltage 49 is also added scaling factor 67. Actual AC linecurrent 64 is compared to AC line current reference 63 to create errorsignal 65. Error signal 65 drives buck stage 60 as part of this servoloop. Current 41 and voltage 42 of PV source 10 are sensed andmultiplied to provide 43, a measure of PV source 10 output power. Acontrol means is used to periodically perturb (45) PV reference voltage46 and observe (44) the effect on PV output power 43. If an increase inPV reference voltage 46 results in an increase in PV power 43, PVreference voltage 46 is incrementally increased every perturb cycleuntil an increase in PV reference voltage 46 results in a decrease in PVpower 43. This is how the maximum power point (MPP) of PV source 10 isestablished. If the irradiance on and the temperature of photovoltaicsource 10 are substantially stable. PV source voltage 42 will dither asmall amount about the MPP of PV source 10. This control method providesstable operation at any PV source operating point. As conditions changethe control circuit will track and seek a new MPP.

FIG. 4 illustrates the preferred embodiment of the DC to AC powerconverter topology. Photovoltaic source 10 is connected at powerconverter input terminals 20 and 21. Capacitor 30 holds the photovoltaicsource 10 voltage substantially constant during the high frequencyswitching cycle of boost circuit 40. Boost circuit 40 is a conventionalpulse width modulated boost circuit comprising inductor 41,semiconductor power switch 43 and diode 45. Boost circuit 40 convertsthe voltage on capacitor 30 to a voltage greater than the peak voltagesof AC line 90. Buck circuit 60 is a conventional pulse width modulatedbuck circuit comprising inductor 61, semiconductor power switch 62 anddiode 63. Boost circuit 40 is a voltage regulator. Buck circuit 60 is acurrent regulator regulating half sinewaves of current, synchronizedwith the voltage of AC line 90. Unfolder circuit 70 is novel. Switch 71is closed when AC line 91 is positive with respect to neutral terminal80 and switch 72 is closed when AC line 92 is positive with respect toneutral terminal 80. Diodes 73 and 74 provide protection against thesimultaneous closure of switches 71 and 72. It may appear that thisarrangement permits DC to flow into the power line, which is limited bypresent interconnection standards. However, the true limitation is toavoid DC flux in the core of the power line transformer. This isaccomplished by the alternate pulsating current in the two lines. Theeffect on the transformer is no different from that of a full-waverectifier. A considerable advantage of this preferred embodiment is thatnegative pole 20 of the photovoltaic source 10 is maintained at thepotential of AC line neutral 80. This greatly improves safety andreduces electrical noise emitted from the PV source 10, minimizinginterference with nearby residential electronic equipment. Theconfigurations for boost circuit 40

or buck circuit 60 can be any circuit that accomplishes the describedresults and are not limited to the circuit configurations shown in FIG.4.

The topology illustrated in FIG. 4 when used with the control methodillustrated in FIG. 1 enables two substantial improvements over theprior art. In the invention, boost stage 40 is allowed to run with itsown independent feedback loop controlled solely for holding the maximumpower point (MPP) of PV source 10. This control is such that there arevery little 60 Hz or 120 Hz current components in capacitor 30.Capacitor 30 must store only enough energy to cover one high frequencyswitching period of boost stage 40. This switching period is roughly1/300th as long as the rectified 60 Hz period. Thus capacitor 30 can bemuch smaller than in a conventional inverter. The second change is toallow the voltage on energy storage element, capacitor 50, to go muchhigher than the peak voltage of AC line 90. Since stored energy isproportional to the square of voltage, any voltage increaseexponentially reduces the capacitance value required of capacitor 50.The value of capacitor 50 can now be in the order of 100 nanofarads perwatt converted. Therefore, both capacitors can be low enough incapacitance value to be economic plastic film units. Also, operation athigh boost ratios also requires some means to constrain the voltage oncapacitor 50 to levels that are safe for semiconductor devices 43, 45,62 and 63. The present invention controls the voltage on capacitor 50 byadjusting the current out of the inverter into AC line 90 whilemaintaining its sinusoidal quality. This topology enables the use of acontrol method with two independent control loops that do not interferewith each other in the presence of rapid changes in the amount of poweravailable from PV source 10 or rapid changes in AC line 90 voltages.

FIG. 5A shows an alternate embodiment of the invention where a bipolarboost circuit and an H-bridge buck circuit are used with the controlmethod illustrated in FIG. 1. Photovoltaic source 10 is connected atpower converter input terminals 21 and 22. Capacitors 31 and 32 hold thephotovoltaic source 10 voltage substantially constant during the highfrequency switching cycles of boost circuit 40. Boost stage 40 is apulse width modulated bipolar boost circuit comprising inductor 41,semiconductor power switch 43 and diode 45 for the positive monopole andinductor 42, semiconductor power switch 44 and diode 46 for the negativemonopole. Boost circuit 40 converts photovoltaic source 10 voltage to apositive voltage on capacitor 51 and a negative voltage on capacitor 52,both with respect to AC line neutral 80 and both substantially greaterthan the respective positive and negative peak voltages of AC line 90.Energy storage element 50, comprising capacitors 51 and 52, storesenergy to limit the voltage excursions across capacitors 51 and 52 asenergy is drawn from energy storage element 50 at twice the AC linefrequency. Buck stage 60 comprises of two half-bridge circuits. Powersemiconductor devices 64 and 65 and inductor 61 are the boost circuitcomponents feeding AC line 91. Power semiconductor devices 66 and 67 andinductor 62 are the boost circuit components feeding AC line 92. Boostcircuit 40 is a voltage regulator. Buck circuit 60 is a currentregulator regulating sinewave currents into AC lines 91 and 92. AC line90 is a typical residential 120/240V, split-phase utility service. Thisarrangement of any conventional, prior art, boost and buck converters isobvious. The amalgamation of this topology and the control methodillustrated in FIG. 1 is claimed as useful, novel and an improvementover prior art.

FIG. 5B shows a variation of the topology illustrated in FIG. 5A wherephotovoltaic source 10 is bipolar with a positive source 11 connectedacross capacitor 31 and a negative source 12 connected across capacitor32 and where all said polarity references are with respect tophotovoltaic common connection point 20 and AC line neutral 80.

FIG. 5C shows a variation of the topology illustrated in FIG. 5A wherebuck stage 60 uses only one half-bridge circuit comprising semiconductorpower devices 64 and 65 and inductor 61. This topology variation isintended for use with single phase AC line 90 rather than thesplit-phase configuration shown in FIG. 5A.

FIG. 6A illustrates a conventional, known DC to poly-phase AC powerconverter topology where photovoltaic source 10 is connected to inputterminals 21 and 22 across the energy storage element, capacitor 50.Buck circuit 60 comprises semiconductor power devices 64 through 69 andinductors 61, 62 and 63 feeding AC lines 91, 92 and 93 respectively.This topology of any conventional, prior art, poly-phase buck converteris known. The amalgamation of this topology and the control methodillustrated in FIG. 3 is claimed as useful, novel and an improvementover the prior art.

FIG. 6B shows a variation of the topology illustrated in FIG. 6A wherephotovoltaic source 10 is bipolar with a positive source 11 connectedacross capacitor 31 and a negative source 12 connected across capacitor32 and where all said polarity references are with respect tophotovoltaic common connection point 20 and AC line neutral 80. Thisconfiguration enables the power converter to be connected to a 4-wire,wye configured AC line 90.

FIG. 7 illustrates a conventional DC to AC power converter topologywhere photovoltaic source 10 is connected to input terminals 21 and 22across the energy storage element, capacitor 50. Buck circuit 60 isconfigured as a full or H-bridge and comprises semiconductor powerdevices 64, 65, 66, 67 and inductor 61. Buck regulator 60 is a currentregulator regulating sinewave current synchronized with AC line voltage90 into primary winding 71 of transformer 70. Transformer 70 providesvoltage isolation and steps up the voltage on primary winding 71 to ahigher voltage on secondary winding 72. This topology of anyconventional buck converter and a step-up, line-frequency transformer isknown. The amalgamation of this topology and the control methodillustrated in FIG. 3 is claimed as useful, novel and an improvementover prior art.

1. An apparatus and electrical power converter topology for convertingthe DC energy from a solar photovoltaic source to AC energy, where saidAC energy is fed into a single phase, split-phase or poly-phase ACelectric utility grid, other AC voltage source or other AC voltage sink,hereafter collectively referred to as the AC load, and comprising; acontrol circuit to provide regulation and protection; a pulse widthmodulated boost circuit to raise a mono-polar DC voltage or bi-polar DCvoltages of said photovoltaic source; and a capacitive energy storageelement to store the output energy of a boost circuit at said raisedvoltage or voltages and a pulse width modulated buck circuit to convertthe energy in said capacitive energy storage element into sinewaves ofcurrent synchronized with an AC load frequency to substantially achieveunity power factor at said AC load and where said control circuit isfurther described as having two servo control loops, one loop thatregulates the voltage or voltages of said photovoltaic source byperturbing or incrementally changing the pulse width of said boostcircuit and observing the resulting change in the power delivered fromsaid photovoltaic source to said capacitive energy storage element andwhere this perturb and observe algorithm dynamically seeks thephotovoltaic source voltage or voltages that will maximize the power outof said photovoltaic source and a second loop that regulates the voltageor voltages of said energy storage element by controlling the pulsewidth modulation of said pulse width modulated buck circuit to adjustthe amplitude of said sinewaves of current sourced into said AC load. 2.An apparatus according to claim 1 where said photovoltaic source iseither monopolar or bipolar.
 3. An apparatus according to claim 1 wheresaid capacitive energy storage element source is either mono-polar orbipolar.
 4. An apparatus according to claim 1 where half sinewaves ofcurrent are created by said buck circuit and an AC unfolder circuit isused as part of said apparatus to steer said half sinewaves of currentinto said AC load with the proper polarity to achieve power transferinto said AC load.
 5. An apparatus according to claim 1 that uses verysmall said capacitive energy storage element capacitance values,substantially in the range of 100 nanofarads per watt converted.
 6. Anapparatus according to claim 1 in which a control means limits themaximum boost ratio of said boost circuit.
 7. An apparatus according toclaim 1 and a control method wherein the photovoltaic source voltage arehigh enough that said boost circuit is not required; only one servocontrol loop regulates the voltage of said photovoltaic source bycomparing the desired operating voltage for the photovoltaic source,designated a PV reference voltage, to the actual photovoltaic sourcevoltage creating an error signal to control said buck circuit andtherefore the amplitude of said sinewaves of current sourced into saidAC load; and the parameter perturbed is said PV reference voltage andthe parameter observed remains the power delivered from saidphotovoltaic source.
 8. An apparatus according to claim 1 or claim 7where the perturb and observe algorithm uses the AC load power or the ACload current as the parameter observed and maximized.